This invention relates to a video phone for transmission of audio signals and freeze-frame video images over voice grade telephone lines.
Video teleconferencing systems typically require dedicated communication channels with a high bandwidth in order to transmit the large amount of data required to represent a complex video picture as in the case of text or graphics. Such devices require a communication channel having a bandwidth in the megahertz (MHz) range. See, for example, U.S. Pat. No. 3,982,063.
Video transmission systems intended to operate over voice grade telephone lines must contend with the fact that the usable telephone bandwidth is only approximately three kilohertz. Because of this bandwidth limitation and a corresponding limitation in the amount of data that can be sent, some of these systems only send a single video frame, rather than a moving picture. In addition, a long time is required for transmission, usually in the range of thirty to sixty seconds or more. See, for example, U.S. Pat. No. 3,842,199, which discloses a video (only) transmission system and U.S. Pat. No. 4,099,202 which discloses a method for multiplexing a slow scan TV signal with a voice signal.
Systems which transmit a single video frame over telephone lines have been touted for their ability to transmit text and graphics such as charts and the like. Accordingly, the resolution of the picture must be sufficiently high to allow a viewer to discern this material. This high resolution requirement results in a long transmission time. Some systems are claimed to be operable with transmission times as low as 8 seconds but with admittedly deteriorated picture quality. Other systems claim transmission times as low as 5 seconds by sending only black and white pixels with no gray scale. However, while a low resolution black and white picture is suitable for very simple graphics, it is not suitable to accurately represent images which have a large grey scale component, such as people's faces.
The long transmission times for video pictures over telephone lines make a telephone unusable for voice communication during image transmission unless a multiplexing system is used. In the simpler, non-multiplexed systems, the user must alternate between using the telephone line for voice communication and using the telephone line for sending video images.
The high transmission speeds are accomplished by a combination of accepting deteriorated picture quality or no gray scale and the use of data compression. Data compression techniques are expensive and complicated to implement. A frequently used method of data compression is run length encoding. For example, if 10 pixels in a row are black, instead of transmitting 10 signals indicating black, one signal indicating black and one signal indicating 10 repetitions are transmitted. Other data compression techniques are more complex.